Higher-order-mode (HOM) fiber has attracted significant interest recently due to the freedom it provides to design unique dispersion characteristics in all-solid silica (non-holey) fibers. This new fiber platform allows for anomalous dispersion below 1300 nm by propagating light solely in one of the higher-order modes. The unique characteristics of the HOM fiber, such as large anomalous dispersion and a large effective area (approximately ten times that of PCFs), provide a number of new opportunities for applications in nonlinear fiber optics. For example, soliton self-frequency shift (SSFS) below 1300 nm could be obtained in an HOM fiber. The advantage of using HOM fiber lies in the ability to generate higher energy self-frequency shifted solitons than attainable in microstructured PCFs. Output pulse energy obtainable for cleanly frequency-shifted solitons in index-guided PCFs is limited to fractions of a nanojoule due to light confinement to a smaller effective area, rendering pulses more susceptible to nonlinearity. In contrast, the HOM fiber platform allows advantages of dispersion curves similar to PCFs, yet with a higher tolerance to nonlinearity. The ability to obtain complex dispersive profiles in fiber is advantageous because of its prospect for realizing sources in hard-to-access spectral regions by exploiting the generation of Cherenkov radiation: that is, the dispersive waves shed by solitons near the zero-dispersion wavelength. HOM fibers, with their higher tolerance to nonlinearities, will allow for energetic sources at wavelengths where sources are not currently available.
Cherenkov radiation in fibers has been demonstrated in microstructured fibers pumped near the zero-dispersion wavelength as well as experiments generating self-frequency shifted solitons. An ideal soliton requires a perfect balance between dispersion and nonlinearity so that energy becomes confined to a discrete packet both spectrally and temporally. With the introduction of perturbations such as higher-order dispersion, this stable solution breaks down, allowing the transfer of energy between the soliton in the anomalous dispersion regime and newly shed dispersive radiation in the normal dispersion regime. Such energy transfer occurs most efficiently in fibers for solitons near the zero-dispersion wavelength. The spectral regime to which energy couples most efficiently has been dubbed “Cherenkov radiation” due to an analogous phase matching condition in particle physics. The phenomenon of Cherenkov radiation in fibers is often associated with soliton self-frequency shift as it allows a convenient mechanism for more efficient energy transfer between the soliton and the Cherenkov band. When the third-order dispersion is negative, soliton self-frequency shift will shift the center frequency of the soliton toward the zero-dispersion wavelength, resulting in efficient energy transfer into the Cherenkov radiation in the normal dispersion regime. A more rigorous description and analytical derivation of Cherenkov radiation in fibers can be found in various theoretical works.
Although Cherenkov radiation can be used in wavelength conversion, the pulse energy is too low for a variety of practical applications. Thus, it would be desirable to use an NOM fiber to produce a fixed output frequency by exciting Cherenkov radiation. This invention is directed to overcoming these and other deficiencies in the art.